1. Field of the Invention
The present invention relates to disk drives. In particular, the present invention relates to a disk drive employing back electromotive force (BEMF) spindle speed control or wedge spindle speed control.
2. Description of the Prior Art
A disk drive typically comprises one or more disks rotated by a spindle motor while heads are actuated radially over the disk surfaces. Each disk surface comprises a number of radially spaced, concentric tracks, where each track is divided into a number of data sectors. A number of embedded servo wedges are also written on each disk surface which facilitate seeking the head and maintaining the head over the centerline of a target track during read and write operations. The disks are rotated at a constant angular velocity (CAV) while varying the data rate from an inner diameter zone to an outer diameter zone to maximize the recording density.
In order to achieve accurate reproduction, it is important to maintain the spindle motor“at speed” while writing data to and reading the data form the disks. To this end, prior art disk drives typically control the spindle speed by monitoring zero crossings in the BEMF voltage generated by the un-energized winding within the spindle motor. However, the bandwidth of a spindle speed control loop using BEMF voltage as feedback may be insufficient for a desired recording density or vibration tolerance.
U.S. Pat. No. 6,067,202 suggests to measure the time between servo sector pulses coincident with detecting each servo wedge, and to generate a speed error by comparing the measured time to a reference time corresponding to the desired spindle speed. Since the servo sector pulses occur more frequently than zero crossings in the BEMF voltage, the bandwidth of the spindle speed control loop increases, allowing for higher recording densities and/or improved vibration tolerance. However, the '202 patent discloses very little implementation details for a spindle speed control loop using servo sector pulses as feedback, and in particular, no implementation details on generating a reliable speed error measurement or recovering from error conditions.
There is, therefore, a need for a disk drive employing a reliable, fail-safe spindle speed control loop using servo wedges as feedback for generating a speed error.